The identification of costimulatory molecules has provided important insights into molecular mechanisms for the regulation of the immune response, and,more importantly, into several novel approaches for autoimmune or tumor immunotherapy. The growing number of known T cell costimulatory pathways, together with the dynamic nature of the immune response, suggests that there may be a functional hierarchy of costimulatory molecules regulating responses of naive, effector, and memory T cells. With the support of the current grant, my laboratory has made significant progress in understanding the role of costimulatory molecules in autoimmune uveitis. However, our studies have also raised two important questions: (1) why are effector uveitogenic T cells more resistant than naive T cells to treatment by costimulatory molecule blockers? (2) Do pathogenic and regulatory T cells rely on different costimulation, so that a specific treatment regimen can be identified to maximally suppress the pathogenic response with a minimal inhibitory effect on regulatory T cell activation? For human disease, immunotherapies that can interfere with an ongoing autoimmune disease are more important than those preventing the development of disease, since disease has already started by the time the patient visits the doctor. Thus, the long-term goal of this proposal is to explore therapeutic approaches inhibiting already activated autoreactive effector T cells. We will therefore determine whether CD28/B7 costimulatory molecules are crucial for the pathogenic effect of uveitogenic T cells, specifically: (1) whether effector and regulatory T cells use CD28/B7 differently in terms of time and quantity and whether blockade of a combination of costimulatory molecules favors the treatment of ongoing autoimmune disease; (2) whether we can identify therapeutic regimens that have a limited impact on regulatory T cell activity, while inhibiting pathogenic activity; and (3) whether a combined treatment acting on costimulation of autoreactive T cell and ocular inflammation can provide better control of ongoing disease. To provide a working model that will allow better understanding of the pathogenesis of recurrent uveitis, we have established chronic, recurrent models in the rat and mouse. These studies should provide insights into the pathogenic mechanism leading to disease progression and help in the development of supplementary therapies for this devastating disease.